1. Field of the Invention
The invention concerns improvements to composite structures with membranes selectively permeable to hydrogen and able to be used in combustible gas processors so as to produce pure hydrogen. It also concerns improvements made to these processors owing to the use of the embodied composite structures.
2. Description of the Prior Art
Generally speaking, so as to obtain a high gas flow through a selective filtering membrane, it is necessary to simultaneously satisfy the following four conditions:                the material making up the membrane needs to be extremely selective and highly permeable to the gas to be extracted;        the membrane also needs to be as thin as possible, the filtered gas flow being an inverse function of its thickness;        the difference of partial pressures of the gas to be removed needs to be as high as possible between the upstream and downstream portions of the membrane, the effectiveness of filtering being directly dependent on this difference of pressures;        the surface of the membrane needs to be as large as possible.        
Moreover, it is known that in the particular case of combustible gas processors, the temperature in the reaction chamber is high (generally between 300 and 600° C.), and that owing to this the only material really effective for embodying a membrane and intended to be worked inside this range of temperatures in palladium or a palladium-based alloy, said material being required for clear economic reasons for reducing the total quantity.
In addition, in the case of easily transportable pure hydrogen generators sought in the motor car industry for producing cars with electric traction equipped with combustible batteries of the proton exchanger membranes type (PEM), it is essential for the required electric powers of between 50 and 100 kW that the total volume occupied by the membrane composite structures, and thus by the reaction chamber where they are installed, is as reduced as possible.
So as to embody membranes as thin as possible yet relatively large, various researchers have suggested producing composite structures constituted by a thin film of palladium or a palladium alloy laid on a permeable rigid substrate resisting to the pressure of the environment.
The U.S. Pat. No. 2,958,391 granted to A. J. Derosset describes a composite structure with membranes selectively permeable to hydrogen, said structure including a thin film of palladium or a palladium alloy laid directly on a porous sintered metal substrate in the form of a plate or elongated cylinder. In principle, only the hydrogen filters through the membrane formed by the thin film and penetrates into the permeable porous substrate connected to a collecting pipe. This type of structure is clearly advantageous when the filtering film is sufficiently thick so as to be really effective and when the sintered substrate possesses sufficient mechanical resistance despite its porosity for satisfying the four conditions mentioned earlier.
However, this type of structure has certain defects. The first is the risk of allowing micro-holes to be formed in the filtering thin film owing to the relatively significant surface roughness of the wall of the substrate. This surface roughness results from the relatively large size of the metal grains used required by the minimum sought-after permeability for the porous substrate. The origin of the second risk lies in the fact that, so as to constitute the substrate, the document does not provide selecting a metal having a heat expansion coefficient compatible with the relatively low heat expansion coefficient (namely 11.8 10−6/° C. for palladium) of the filtering film. This metal is required so as to allow full selectively of the permeability of this film, by provoking microcracks due to prejudicial differential expansions.
In addition, where the gas mixture including the hydrogen to be filtered is subjected to high pressure and a high temperature, the six conditions mentioned above are insufficient for a structure including a filtering film laid on a substrate. In fact, it is also essential that at the high temperatures and pressures in question (generally 300 to 600° C. and 5 to 15 bars), the metal of the filtering film does not diffuse inside the metal of the substrate which would significantly reduce the selective permeability of the membrane with respect to the hydrogen. This also means that it is essential that the two metals in contact are chemically stable with respect to each other at the temperatures and pressures concerned.
The U.S. Pat. No. 5,498,278 granted to D. J. Edlund proposes a solution to these various problems. Here, the composite structure described includes three elements: (1) a flexible porous intermediate film, non-sintered, textured and heat and chemically stable, is placed between (2) a thin metal film selectively permeable to hydrogen, such as palladium or a palladium, silver and/or nickel alloy, and (3) a permeable rigid substrate. The intermediate film in question is a woven or non-woven film made for example of aluminium, silicon, glass or carbon fibres. It totally separates the external filtering film from the internal permeable rigid substrate and renders them totally independent of each other. Any consideration of direct compatibility, especially chemical or thermic, between the material of this filtering film and that of the substrate is in principle eliminated as virtually useless owing to the presence of this particular intermediate film serving as a barrier. Thus, the substrate could be more of less any and for example made of full metal or dense ceramic material rendered permeable by cuts or perforations. In this structure and according to the document, the filtering film and the flexible intermediate film possess maximum efficiency when they comprise microwaves in two orthogonal directions enabling them to operate as micro-blowers adapted to absorb any differential movement with respect to the substrate. However, these arrangements have one major drawback which is a direct consequence of the non-metallic nature of the intermediate film. Indeed, this renders impossible any genuine weld of the metallic filtering film and the intermediate film which is not so. The reciprocal fixing of these two films having different natures can only be a sort of glueing with relative stability and effectiveness. In these circumstances, at the end of a relatively short period of use comprising successive periods of functioning and stoppage, the filtering film, which undergoes relatively significant heat contractions and expansions and with regard to those (virtually nul) of the intermediate film, shall inevitably come away from its support and shall quickly become fragile, then cracked and finally non-operative. An identical situation would occur if the body of the substrate and the intermediate film or either or both were made of a ceramic material, namely being fragile and brittle.